The patterning of a silicon film (such as an amorphous silicon film or a polycrystal silicon film) used with integrated circuits and thin film transistors is generally carried out through a process in which a silicon film is formed over the whole surface by a vapor-phase process such as CVD (Chemical Vapor Deposition) and then unwanted parts are removed by photolithography.
However, this method has problems such as a bulky apparatus being required due to use of the vapor-phase process, the use efficiency of the raw material being poor, the raw material being difficult to handle as it is gaseous, and a large amount of waste being produced.
To cope with these problems, studies into a method of forming a silicon film by a liquid-phase process are now under way. For example, there is proposed a method of forming a silicon film by applying a high order silane composition comprising a liquid silane compound (such as cyclopentasilane), a high order silane compound obtained by photopolymerizing this liquid silane compound by irradiation with ultraviolet light, and an organic solvent such as decalin, tetralin, methyl naphthalene, toluene, decane, octane, xylene or benzene onto a substrate, removing the solvent, and heating the composition (refer, for example, to JP-A 2003-313299).
However, the above high order silane compound has low solubility in the solvent used in this high order silane composition. That is, when only the above organic solvent is used in this high order silane composition without using the liquid silane compound, it cannot dissolve the high order silane compound having a molecular weight high enough to form a silicon film having a desired thickness in a sufficiently high concentration. However, the high order silane compound is soluble in the above liquid silane compound (low order silane compound) which is soluble in the above solvent. Therefore, in the high order silane composition disclosed by JP-A 2003-313299, the high order silane compound is made coexistent with the low order silane compound so that the high order silane compound is dissolved in the solvent. That is, it can be understood that the high order silane compound contained in the high order silane composition disclosed by the above publication is dissolved in a mixed solvent of the above solvent and the low-order silane compound.
To increase the molecular weight and content of the high order silane compound in this high order silane composition, the content of the low order silane compound must be increased. However, as the low order silane compound has high reactivity with oxygen and the vapor pressure is high, the mixed solvent becomes unstable.
In view of the above situation, the development of a high order silane composition having high stability and excellent safety, and comprising a high order silane compound having a sufficiently high molecular weight and a sufficiently high concentration to form a high-quality film having a desired thickness is desired.